It commonly has been assumed that photoreceptors respond independently of one another and that visual processing involving synaptic interactions between neurons occurs at more proximal locations in the visual system. Since it recently has been established that photoreceptors do interact synaptically with one another, it is important to understand the function and physiological processes of these interactions. The project proposed herein investigates the pathways, mechanisms and functional significance of the synaptic interactions which influence the rod responses and cone responses in the retina of the snapping turtle. An electrophysiological approach will be used to record the intracellular potentials of individual rods, cones and horizontal cells. The nature and basis of the summative interaction between rods and between cones and the antagonistic interaction between horizontal cells and cones will be determined. Connectivity between pairs of retinal neurons will be investigated by impaling two cells simultaneously. Membrane properties of photoreceptors will be studied with double-barrelled microelectrodes. This study will establish the specific rod-rod and cone-cone pathways and will determine how the characteristics of the neuronal circuitry influence the coupling between these cells. This study will also investigate the horizontal cell to cone feedback interaction, first by identifying the specific classes of horizontal cells and cones which interact, and then by identifying the specific chemical transmitters and ionic conductance changes that are involved in this interaction.